We hypothesize that HIV protease inhibitors alter macrophage class B scavenger receptor-dependent uptake and efflux of cholesterol thereby promoting the formation of lipid-laden macrophages and atherosclerotic lesions. A major drawback to the use of HIV protease inhibitors is that they promote the development of dyslipidemia, which is an established risk factor for the development of atherosclerosis. Numerous reports have suggested a causal link between protease inhibitor therapy and atherosclerosis; however, this has not been unequivocally demonstrated in a large-scale clinical trial. The dyslipidemia, which is primarily an increase in triglycerides, is unlikely to completely account for the development of atherosclerotic lesions in HIV patients because atherosclerosis is a multifactorial disease that is not controlled by a single factor. Our preliminary data demonstrate that HIV protease inhibitors have direct effects on macrophages, which are critical cellular mediators in atherosclerotic lesion development. The generation of lipid-laden macrophages is a key event in atherogenesis and is thought to be due, in part, to unregulated uptake of modified lipoproteins. Such aberrant cholesterol accumulation is influenced by the functions of the class B scavenger receptors, SR-BI and CD36. Both receptors are found in atherosclerotic lesions and on macrophages. In addition, both receptors can mediate the uptake of lipoprotein cholesterol and the efflux of cellular cholesterol. Our preliminary data demonstrate that peritoneal macrophages isolated from LDL receptor null mice given the HIV protease inhibitors, amprenavir, indinavir, or ritonavir, contain more SR-BI and CD36 than aged-matched controls. In addition, all three protease inhibitors increased SR-BI and CD36 levels in THP-1 cells, our macrophage cell model system. The protease inhibitors also increased the cellular cholesterol content in both the in vivo and in vitro model systems, which is consistent with our hypothesis. Importantly, mice given amprenavir, indinavir, or ritonavir had significantly more atherosclerotic lesions than control mice. We will test two Specific Aims. Aim 1 : To determine the effects of HTV protease inhibitors on SR-BI and CD36 dependent cholesterol uptake and efflux. Aim 2: To determine the leukocyte (i.e., macrophages, etc.) specific effects of HIV protease inhibitors on atherosclerotic lesion formation in LDL receptor null mice that have been transplanted with bone marrow from SR-BI x LDLR and CD36 x LDLR null mice.